“Brake-by-wire” braking systems are being used ever more frequently in automotive technology. Braking systems of this type frequently comprise in addition to a master brake cylinder that can be actuated by the vehicle driver, an electrically controllable pressure generating device by means of which in the operating mode “brake-by-wire” the wheel brakes or the master brake cylinder are actuated. In order to provide the vehicle driver with a pleasant pedal sensation in the “brake-by-wire” operating mode, the braking systems generally comprise a brake pedal sensation simulation device. In the case of these braking systems, the wheel brakes can also be actuated in response to electronic signals without any active input by the vehicle driver. These electronic signals can be emitted for example by an electronic stability program or by a proximity control system.
A “brake-by-wire” braking system is known from the international patent application WO 2011/029812 A1 having a pressure generating device comprising a hydraulic cylinder-piston arrangement, the piston of which can be actuated by an electric motor whilst inter-positioning a rotation-translation gear. The pressure generating device is connected hydraulically to a pressure modulation unit that comprises for each wheel brake an inlet valve and an outlet valve. The braking system comprises a pressure sensor for measuring the pressure (braking system pressure, pre-pressure) of the pressure generating device. The previously referenced application does not contain any more detailed explanations as regards how to set or reset a predetermined desired value for the pressure of the pressure generating device.
The unpublished document DE 10 2011 076 675 discloses a method for controlling an electrohydraulic braking system having an electrically controllable pressure generating device that comprises a cylinder-piston arrangement, the piston of which can be actuated by means of an electromechanical actuator. In order to set a suitable pressure in the cylinder-piston arrangement, in the case of an antilock braking control operation the pressure is always set higher by a safety margin ΔP than the desired wheel braking pressure at the wheel brake having the highest pressure requirement. As a consequence, it is achieved that all wheels being subjected to an antilock braking control operation are sufficiently charged so that it is possible to perform a pressure increase procedure at each wheel by opening the associated inlet valve. In order to maintain a constant pressure at a wheel, the inlet valve is actively closed, whereas the outlet valve remains closed. During a pressure reduction procedure, the inlet valve is closed, whereas by opening the outlet valve for a short period of time pressure medium is discharged from the wheel brake.
The control method described in DE 10 2011 076 675 has the principle disadvantage that it is difficult to set the pressure in the pressure chamber of the piston-cylinder arrangement with a high level of accuracy since in the by far longest time periods of the control operation all inlet valves are closed and consequently the pressure chamber are separated from the relatively elastic wheel brakes. In order to be able to modulate the pressure in a rapid and reproducible manner, the pressure chamber itself is not to have any noticeable elasticity. However, from the point of view of control technology, this boundary condition represents a problem, since even an extremely small modulation of the piston travel causes the pressure in the rigid pressure chamber to fluctuate by amounts that are not insignificant. In addition to the problems associated with control technology, the above described control operation also produces particular noise problems. The higher the pressure differential across the inlet valves, the greater are the switching noises if a valve is switched to through-flow. The constant switching on and off of the wheel brakes by opening and closing the inlet valves therefore has a permanent adverse effect when controlling the pressure in the pressure chamber and disturbing noises occur.
It is therefore the object of the present invention to provide an electrohydraulic braking system having an electrically controllable pressure generating device that comprises a cylinder-piston arrangement, the piston of which can be actuated by means of an electromechanical actuator, and a method for controlling said braking system, which method allows a low-noise antilock braking control operation to be performed. In addition, a high quality control operation is to be achieved.
This object is achieved in accordance with the invention by means of a method in accordance with claim 1 and a braking system in accordance with claim 14.
The invention is based on the idea, when performing an antilock braking control operation on the wheel brakes that are connected to the hydraulic pressure chamber of the pressure generating device, of setting as the pressure in the hydraulic pressure chamber precisely that pressure required by the wheel brake having the highest to-be-set wheel desired pressure.
Preferably, the pressure in the hydraulic pressure chamber during the antilock braking control operation always corresponds to the respective highest wheel desired pressure of the wheel brakes.
In accordance with a further development of the invention, during the antilock braking control operation always at least one wheel brake having the highest wheel desired pressure in terms of full pressure compensation between the wheel brake and the pressure chamber is connected to the hydraulic pressure chamber.
It is preferred during the antilock braking control operation that the inlet valve of a single wheel brake having the highest desired wheel pressure is open so that the pressure in the hydraulic pressure chamber is equal to the wheel braking pressure of this wheel brake.
In the case that the same highest wheel desired pressure or approximately the highest wheel desired pressure is to be set at two or more wheel brakes, preferably the inlet valves of all these wheel brakes are open during the antilock braking control operation.
The invention provides the advantage that the pressure chamber during the antilock braking control operation is connected to at least one relatively elastic wheel brake. As a consequence, the pressure control operation is simplified as far as the control technology is concerned. In addition, a switching of the inlet valve of the wheel brake that is connected to the pressure chamber is avoided and the switching noises of the other inlet valves are reduced on the basis of the lower pressure differential across the inlet valves.
The pressure generating device comprises essentially a hydraulic cylinder-piston arrangement and an electromechanical actuator that is preferably embodied by an electric motor having a reduction gearing.
The electromechanical actuator is preferably embodied in a manner such that it is able to reverse and ensures a translation movement of a hydraulic piston of the cylinder-piston arrangement, so that a hydraulic pressure can be increased and reduced in a pressure chamber of the hydraulic cylinder-piston arrangement. A rotation position sensor is advantageously used to sense the rotation position and the travel distance of the electromechanical actuator.
Preferably, pressure increase procedures and/or pressure reduction procedures are to be performed on the wheel brake having the highest wheel desired pressure or on the wheel brakes having the highest or almost highest wheel desired pressure are performed exclusively by means of displacing the piston of the cylinder-piston arrangement as long as the wheel desired pressure of the wheel brake (n) is the highest wheel desired pressure. As a consequence, the switching of the inlet and outlet valves for the pressure modulation operation and consequently switching noises are avoided.
In accordance with a preferred embodiment of the method in accordance with the invention, each wheel brake whose wheel desired pressure does not correspond to the highest wheel desired pressure is separated by a closed inlet valve from the hydraulic pressure chamber.
It is further preferred that the inlet valve of the wheel brake whose wheel desired pressure does not correspond to the highest wheel desired pressure is opened for a short period of time in the case that pressure is to be increased at the wheel brake and that the outlet valve of the wheel brake whose wheel desired pressure does not correspond to the highest wheel desired pressure is opened in the case that pressure is to be reduced at the wheel brake.
The wheel desired pressure for a wheel brake is preferably equal to an ABS pressure request that is determined by means of the antilock braking control function if the ABS pressure request is smaller than a braking pressure that is requested by a vehicle driver. On the other hand, the wheel desired pressure for a wheel brake is preferably equal to the braking pressure requested by the vehicle driver if the braking pressure requested by the vehicle driver is smaller than the ABS pressure request. As a consequence, the wheel desired pressure is limited to the braking pressure requested by the vehicle driver, e.g. if the driver abandons the brake pedal during the antilock braking control operation.
The ABS pressure request that is determined by means of the antilock braking control function is advantageously equal to a predetermined maximum pressure value if the wheel is not being subjected to an antilock braking control operation or the antilock braking control operation does not have priority at this wheel.
In accordance with a further development of the invention, the inlet valves are embodied as analog inlet valves or as inlet valves that operate in an analog manner.
Preferably, a pressure increase procedure in a wheel brake whose wheel desired pressure does not correspond to the highest wheel desired pressure is performed by means of digitally controlling the associated inlet valves if the pressure differential between the pressure in the hydraulic pressure chamber and the actual wheel desired pressure is smaller than a predetermined threshold value.
It is likewise preferred that the hydraulic pressure chamber and/or the hydraulic pressure chambers of the cylinder-piston arrangement is/are connected by means of at least one valve, advantageously a non-return valve, to the pressure medium storage container, so that by means of drawing back the piston of the cylinder-piston arrangement pressure medium can be drawn from the pressure medium storage container into the pressure chamber/pressure chambers.
Advantageously, the method in accordance with the invention is performed in a braking system for motor vehicles, which braking system can be controlled in a so-called “brake-by-wire” operating mode both by the vehicle driver and also independently from the vehicle driver, which braking system is preferably operated in the “brake-by-wire” operating mode and can be operated in at least one fallback operating mode in which only the operation by means of the vehicle driver is possible.
In accordance with a further development of the invention, the inlet valve of the wheel brake (n) having the respective highest desired pressure value is continuously open and the outlet valve of the wheel brake (n) having the respective highest desired pressure value is continuously closed, wherein advantageously the inlet valves are embodied as de-energized open valves and the outlet valves are embodied as de-energized closed valves.